1. Field of the Invention
The present invention relates to a photocathode that emits photoelectrons in response to incidence of light with a predetermined wavelength, and a photomultiplier and an electron tube each including the same.
2. Related Background Art
A photocathode is, as described in, for example, U.S. Pat. No. 3,254,253, a device that emits electrons (photoelectrons) generated in response to an incident light. Such a photocathode is favorably applied to an electron tube such as a photomultiplier. In addition, the photocathode can be of two types: transmissive and reflective, according to the difference in supporting substrate materials to be applied thereto.
In a transmissive photocathode, a photoelectron emitting layer is formed on a supporting substrate comprised of a material that transmits an incident light, and a part of a transparent container of a photomultiplier or the like functions as the supporting substrate. In this case, when an incident light transmitted through the supporting substrate reaches the photoelectron emitting layer, photoelectrons are generated within the photoelectron emitting layer in response to the reached incident light. As a result of an electric field for a photoelectron extraction being formed on the side opposite to the supporting substrate when viewed from the photoelectron emitting layer, the photoelectrons generated within the photoelectron emitting layer are emitted toward a direction coincident with a traveling direction of the incident light.
On the other hand, in a reflective photocathode, a photoelectron emitting layer is formed on a supporting substrate comprised of a material that blocks an incident light, and the supporting substrate is arranged inside a transparent container of a photomultiplier. In this case, the supporting substrate functions as a reinforcing member to support the photoelectron emitting layer, and an incident light directly reaches the photoelectron emitting layer while avoiding the supporting substrate. Within the photoelectron emitting layer, photoelectrons are generated in response to the reached incident light. The photoelectrons generated within the photoelectron emitting layer are, as a result of an electric field for a photoelectron extraction being formed on the side opposite to the supporting substrate when viewed from the photoelectron emitting layer, emitted to the side from which the incident light has traveled and reached when viewed from the supporting substrate.